Question: In math class, the girl to boy ratio is $3$ to $6$. If there are a total of $45$ students, how many girls are there?
A ratio of $3$ girls to $6$ boys means that a set of $9$ students will have $3$ girls and $6$ boys. A class of $45$ students has $5$ sets of $9$ students. Because we know that there are $3$ girls in each set of $9$ students, the class must have $5$ groups of $3$ girls each. There is a total of $15$ girls in math class.